Displays, keyboards, and touch pads which detect a pressure exerted on them are known. Such devices include pressure sensors comprising a layer of piezoelectric material formed on a flexible support. When it is pressed on these devices, the piezoelectric material layer of the pressed sensor(s) deforms and releases electric charges which are detected by a dedicated electric circuit. The detection of the released charges then determines that a pressure is exerted on the device.
To obtain high-quality pressure sensors, piezoelectric materials which have both good mechanical qualities and good piezoelectric qualities are usually searched for.
It is in particular desired for these materials to have a good elasticity, a good adherence to the flexible support, as well as a low thermal expansion difference with the support. Indeed, too rigid a piezoelectric material is difficult to install on a flexible support, and if the thermal expansion coefficient difference is too large, the piezoelectric layer may separate from the support under the effect of temperature variations. The resulting pressure sensors are thus mechanically fragile. Piezoelectric materials having a high piezoelectric constant, that is, releasing a large quantity of charges when pressed, even slightly, are also searched for, to design sensors of high sensitivity.
Now, current piezoelectric materials have either good mechanical properties, or good piezoelectric properties. For example, piezoelectric materials currently used for pressure sensors comprise so-called “PZT” ceramics (acronym of lead titanium zirconium) and PVDF (acronym of polyvinylene fluoride).
However, although a PZT ceramic has a high piezoelectric constant, it has a very high Young's modulus, and thus insufficient mechanical properties. As for PVD, it has a low Young's modulus and more generally good mechanical properties, but however has a low piezoelectric constant.
For reasons of mechanical robustness, piezoelectric materials having a good mechanical behavior, and thus a low Young's modulus, are usually preferred. However, due to the low piezoelectric constant, and thus to their low response to an applied pressure, a so-called “capacitive” electric assembly is used to measure the quantity of released charges. In such an assembly, the piezoelectric material layer is in contact with electrodes and forms therewith a capacitive circuit, having its capacitance varying according to the released electric charges. A capacitance meter, connected to the electrodes, dynamically measures the capacitance of the capacitive circuit, particularly by imposing a variable voltage between the electrodes and/or by injecting therein a variable current of variable frequency. A capacitive electric assembly thus requires complex measurement means.